FIG. 9 shows a conventional cooling device wherein an engine 301 and a radiator 302 are connected to each other by conduits 304 through which a cooling fluid for cooling the engine 301 is circulated by a water pump 303. A bypass conduit 305 is connected to the conduits 304 at both an inlet portion and an outlet portion of the radiator 302. When the temperature of cooling fluid flowing out of the radiator 302 is above a predetermined value, the cooling fluid flows through bypass conduit 305 to bypass the radiator 302. When the temperature of the cooling fluid is below the predetermined value, a thermostat valve 306 closes the bypass conduit 305 so that the cooling fluid flows into the radiator 302 to be cooled. A heater core 308 is provided in the conduit 304. In order to cool the engine 301 efficiently, it is required that the cooling efficiency of the cooling device be controlled according to the condition of the engine 301, which varies frequently. The water pump 303 is driven by the engine 301 and the discharge capacity of water pump 303 is determined so as to prevent cavitation of the cooling fluid in the water pump 303 and to circulate plenty of cooling fluid even under extreme conditions, for instance, where the automobile climbs a slope at low speed.
Recently, engines have become more powerful and transmit more heat to the cooling fluid. Therefore, the radiator and a cooling fan are required to be large enough to radiate the heat efficiently. However, the engine compartment has become increasingly smaller, making if harder to use large radiators and cooling fans. One idea to radiate the heat more efficiently is to make the discharge capacity of the water pump larger. However, the increment of the discharge capacity of the water pump causes cavitation when the water pump rotates at high speed, and a loss of power due to the water pump when cooling requirements are lower. Therefore, increasing the discharge capacity of the water pump is not practical and it is hard to increase the flow rate of circulating cooling fluid under a condition of low rotation and high load of the engine.
Japanese unexamined utility model (Kokai) 63-190520 shows a cooling device which has an additional water pump 320 beside the main water pump 303 as shown in FIG. 10. Since the main water pump 303 is driven by the engine 301, the discharge volume of the main water pump 303 varies frequently according to the revolutions per minute (r.p.m.) of the engine. The shortage of cooling fluid or the surplus of cooling fluid arises under certain conditions of the main water pump 303 and the additional water pump 320. Sufficient cooling fluid is not supplied according to the engine rotation and load by merely providing the additional water pump 320.
FIG. 3 shows the relation between the r.p.m. of the water pump and the discharge volume (flow rate) thereof. The flow rate of the main water pump increases in proportion to the r.p.m. as shown by line A in FIG. 3. When the r.p.m. is low, which means that the automobile is climbing a slope at low speed or the engine 301 is idling, the flow shortage of the cooling fluid becomes apparent. The total flow of the main water pump 303 and the additional water pump 320 is represented by broken line C, which shows that the flow is not increased tremendously. The reason why the sufficient increment of flow is not achieved is that the cooling fluid discharged from the additional water pump 320 recirculates into the inlet of the additional water pump 320 through the bypass conduit 330. Such a short-circuit of the cooling fluid can be prevented by providing a one way valve 331 in the bypass conduit 330.
Since the one way valve 331 has a flowing resistance, the amount of cooling fluid flowing in the bypass conduit 330 and the additional water pump 320 is determined, based on the flowing resistance of the way valve 331 and the additional water pump 320. In other words, even if the engine 301 rotates at high speed and the pumping operation of the additional water pump 320 is not necessary, a certain amount of the cooling fluid flows into the additional water pump 320 according to the resistance of the one way valve 331. The resistance of the one way valve 331 also restricts the flow of the cooling fluid discharged from the main water pump 303. The resistance of the one way valve 331 is not variable according to the heat load of the engine 301.
As described above, the conventional cooling device does not operate well according to the frequently varying condition of the engine.